Sleep disordered breathing, such as Obstructive Sleep Apnea (OSA), is treated with devices which provide positive pressure ventilatory assistance, such as Continuous Positive Airway Pressure (CPAP) devices. A typical device comprises a controllable flow generator coupled to a nasal mask that provides a supply of breathable gas to a patient in the range 4 to 30 cm H2O positive pressure. Furthermore, a field of ventilatory assistance known as non-invasive positive pressure ventilation (NIPPV) supplies a patient with various pressures at appropriate times during the patient's breathing cycle. Throughout this specification a reference to “CPAP” or “ventilatory assistance apparatus” is to be understood as including a reference to CPAP or non-invasive positive pressure ventilation. Nasal prongs, a mouth mask or full face mask may be used as alternatives to a nasal mask. A reference to a mask herein is intended to include a reference to any of these patient interface devices.
Apparatus, and thus treatment efficacy depends on correct mask fitting to reduce or eliminate leaks. A known arrangement of CPAP apparatus (for example) provides a test mode, which may be used prior to the functional (or operational) mode, whereby the user can test-fit the mask. Whilst in this test mode, the apparatus provides a test pressure. This test pressure may be, for example, fixed at 10 cm H2O or alternatively the maximum output pressure of the device. During this test mode the mask is fitted to the patient so as to avoid leaks that might occur at the test pressure.
Another known arrangement is that the mask test pressure is chosen to be a function of the minimum and maximum pressure settings. For example:Mask-fit test pressure=Pmin—set+0.75(Pmax —set−Pmin—set)  (1)where
Pmin—set=Minimum pressure setting
Pmax—set=Maximum pressure setting
The problem with these known methods is that the test pressure will be independent of the pressures actually delivered during an automatically titrating mode. During the automatically titrating mode (or autotreatment mode) the device varies the pressure delivered in the mask in accordance with the patient's requirements while the patient sleeps. Examples of devices and methods of treatment that operate in such an automatic mode can be found in commonly owned U.S. Pat. No. 5,704,345 (Berthon-Jones assigned to ResMed Limited) and WO 98/12965 (Berthon-Jones assigned to ResMed Limited).
A good indication of mask fitting under normal conditions of use will not be obtained if the test pressure is significantly different to the pressures encountered in normal use. No leaks may be detected during the test mode, but during the functional mode, the mask may leak. Alternatively, the mask-fit test pressure may be unnecessarily high and discourage the patient from using the mask-fit feature, or from using the device due to the discomfort resulting from the test pressure suggesting that the patient fit the mask with a strap tension that is greater than would be necessary in practice.
FIG. 1 shows a cumulative frequency plot for overnight treatment using an automatically titrating CPAP device for two patients, (A′_′,B′-.-.-′), together with the 95th percentile (------). The sampling rate was 1 per minute. For patient A, there were 375 pressure readings and the 95th percentile pressure was approximately 7 cmH2O. For patient B, there were 79 pressure readings and the 95th percentile pressure was approximately 20 cmH2O. If the prior art solution using equation (1) had been adopted, with the 4 cm and 20 cm minimum and maximum pressure settings respectively, the mask-fit pressure would have been 16 cm H2O. This pressure is much higher than necessary for patient A, and may have been insufficient for patient B.